ground testing methods

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sguinn

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What's a good method for testing resistance? I've heard that a clamp-on tester is the easiest. Anyone have any ideas or where I might get one? This would be for mostly residential and I would appreciate any pictures, thanks.
 
My first question after reading this type of question is WHY? Or more accurately, WHAT do you expect to learn from this information?

The best "test" of a grounding electrode system is to simply ensure the installation meets the minimum code requirements, has properly tightended and secured clamps and connections, and has properly listed materials of good quality and duribility.

Beyond that, there is no test that will give you any more assurance or indication that the system will operate properly. Knowing the grounding resistance will not provide with any indication of any practical use.

For example, say you install a grounding system on a service that measures 500 ohms. Is that bad? Will that be okay? What exactly does that mean and what, if anything, should be done.

Say the next installation you make has a reading of 5 ohms. Is that good. Does that make things better? Will the system operate better than the 500 ohm one?

Bottom line, do you think lightning or other surging events will know the difference?

I know how I would answer all these quetions...
 
the 3 point fall of potential method is a true classic. unless someone specifically requests a ground resistance test there is no reason to do it. the 25 ohm min resistance required by code is not enough to shut off a breaker. if you put 120 volts to a 25 ohm ground rod or any other electrode it will only draw about 4.8 amps and wont shut the breaker off because of the earths high resistance. bph hit the nail on the head with his post. the best test is just to make sure you have the correct wire and equipment and all that.
 
TO properly perform this test it is more than a simple drive a few test probes. For residential if properly applied the clamp on meter should be ok. Check AEMC, Fluke and AVO websites.
 
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bphgravity said:
My first question after reading this type of question is WHY? Or more accurately, WHAT do you expect to learn from this information?



Bottom line, do you think lightning or other surging events will know the difference?

I know how I would answer all these quetions...
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I simply want to do the best job that I can, and other than guessing as to what the resistance is I figured some sort of instrument would be the proper method. If a ground rod doesn't have 25 ohms then another is required. If I simply start requiring ECs to drive two ground rods and they ask "how many ohms does the one have?" I just don't want to stand there with my thumb you know where.
 
I agree with Brian. Usually I advise against clamp on ground testers because they have limitations on when they can be used, have questionable accuracy, and are usually misused becuase the user dosent understand the operation of the equipment.

I also believe that the value dosent matter for residential (LV) applications, so you are going above the call of duty to test it (Good for you). Since accuracy is not an issue (20 Ohms, 10 Ohms, you dont care) then a clamp on meter might be the way to go for you.

If you want a true accurate reading, go with the fall of potential test.

Brian listed several good references for meters.
 
sguinn said:
I simply want to do the best job that I can, and other than guessing as to what the resistance is I figured some sort of instrument would be the proper method. If a ground rod doesn't have 25 ohms then another is required. If I simply start requiring ECs to drive two ground rods and they ask "how many ohms does the one have?" I just don't want to stand there with my thumb you know where.
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What does it matter? It would be an extremely rare occurance a single rod would ever approach 25 ohm's. More like 500 to 1000. Drive the second rod, call it a day, and save $5000 you would spend on the meters and an hour or two of your time. Work smart. :cool:

As stated it doesn't make a flip what the earth impedance is, it is a useless number in a residence application, and most other applications.

The only accurate measurement is a 3-point Fall-of Potential test, and that takes an hour or two to do right. The AEMC Clamp-On and the like types are not accurate and always read higher then the unit under test. Drive the second rod and go home.
 
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Dereck:

Inquiring minds want to know, I am what I am, because of this philosophy in electricity (with hold inserting sarcastic comments here PLEASE). I ALWAYS wanted to know. So I purchased magazines, books and test equipment to KNOW the answer.
 
sguinn,

I suggest to visit MEGGER website (www.megger.com). They have got plenty of information about equipments and very interesting papers related to ground resistance testing, try to get the one named "Getting Down to Earth", that paper will clarify the most of your doubts.
 
The rough thing about using much of Megger's equipment for checking ground rod resistance is that they take averaging type readings. There's lots of ways to check ground rod resistance. Some quite accurate, and some not so much.
 
brian john said:
Dereck:

Inquiring minds want to know, I am what I am, because of this philosophy in electricity (with hold inserting sarcastic comments here PLEASE). I ALWAYS wanted to know. So I purchased magazines, books and test equipment to KNOW the answer.
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Brian you are right and I appologize.
 
brian john said:
Dereck:

Inquiring minds want to know, I am what I am, because of this philosophy in electricity (with hold inserting sarcastic comments here PLEASE). I ALWAYS wanted to know. So I purchased magazines, books and test equipment to KNOW the answer.
Click to expand...
absolutly the best post posted by anyone on earth. huge moral boost. mike holt talks about the fall of potential test method in his new grounding and bonding 2008 book. if you want to learn more about grounding buy that book and read it.
 
Useless knowledge for the sake of knowledge is useless.

I have likely spent more time and energy on the history of the 25-ohm value established in 250.56 than anyone else on this Forum. I have a 2in, 3-ring binder stuffed with papers and doucments exclusively on this topic.

There may be a fine line between the difference in trying to understand the history of the code and the purpose of a section verses actually performing work to obtain unhelpful or meaningful data. But, there is a difference.

I have a clamp on ground resistance meter. I had a great time running all over the city testing electrode systems. But I learned nothing. I am unable to take the information I did get from these experiments to do my job better or to give contractors information to do their work better.

WHY? Because ground resistance testing isn't required nor gives any indication of anything.

I am not trying to come off as negative, though it may appear that way. This just happens to be a subject I am quite passionate about.
 
sguinn said:
What's a good method for testing resistance? I've heard that a clamp-on tester is the easiest. Anyone have any ideas or where I might get one? This would be for mostly residential and I would appreciate any pictures, thanks.
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The only necessary test is to make sure that your GES meets the minimum code requirements.
 
Ok here's another question. Who's responsibility would it be to check, EC, Poco or inspector? If I required a second ground rod and the EC asked why would I ask him if HE had checked the resistance. 250.56 just says if the resistance isn't 25 ohms or less, it shall be augmented by one additional electrode. THEN, what if I'm asked "how do you know it doesn't have 25 ohms or less"? I'm just trying to be fair and code compliant, I have my own rear to cover as well.
 
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sguinn said:
Ok here's another question. Who's responsibility would it be to check, EC, Poco or inspector? If I required a second ground rod and the EC asked why would I ask him if HE had checked the resistance. 250.56 just says if the resistance isn't 25 ohms or less, it shall be augmented by one additional electrode. THEN, what if I'm asked "how do you know it doesn't have 25 ohms or less"? I'm just trying to be fair and code compliant, I have my own rear to cover as well.
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I can't say about other areas in our great nation but here in NC it is the electrical contractor that has the burden to show 25 ohms or drive the second rod.
The electrical code official only has to ask the electrical to show that there is 25 ohms not the electrical official to do the testing
 
jwelectric said:
I can't say about other areas in our great nation but here in NC it is the electrical contractor that has the burden to show 25 ohms or drive the second rod.
The electrical code official only has to ask the electrical to show that there is 25 ohms not the electrical official to do the testing
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It may be the way inspectors approach the problem, but it may also not be something that is supported by law. Put the second rod in and be done with it.
 
petersonra said:
It may be the way inspectors approach the problem, but it may also not be something that is supported by law. Put the second rod in and be done with it.
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I can choose to avoid the burden of testing I can add the additional rod and be done with it.

OTOH if I only drive one rod and expect the inspector to accept it IMO the burden is on me to prove the 25 ohms.
 
Squinn, I agree with the others. When you arrive at a job and the EC has only installed one rod, he had better have the test set ready to show you he meets the 25 ohm requirement.

OTOH, if you show up and there are two rods driven, you and the EC move on to other items or jobs.

Roger
 
I have likely spent more time and energy on the history of the 25-ohm value established in 250.56 than anyone else on this Forum. I have a 2in, 3-ring binder stuffed with papers and documents exclusively on this topic.
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I have several loose leaf binders, aptly titled "GET GROUND WITH YOU BAD SELF". I never researched the 25 ohm rule but will read any information surrounbding this subject if it is made available. I want to know as much about, grounding, testing grounds and resolving ground issues. In my end of the business some folks call me a expert (?), I attempt to know as much about the subject as I can.
 
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